The Fischer-Tropsch process is a chemical process of increasing relevance as a source of middle distillates from which transportation fuels, including diesel fuel, gasoline and aviation fuels, are derived. It is especially important as natural sources of these middle distillate fractions, namely crude petroleum, are dwindling with world reserves expected to near depletion within a century or less. The Fischer-Tropsch process comprises contacting a reactant gas mixture comprising carbon monoxide and hydrogen, called synthesis gas or syngas, with a catalyst in a suitable reactor and under suitable conditions of pressure and temperature to produce a product mixture comprising hydrocarbons, carbon dioxide and/or water; the outcome depending, in part, on the hydrogen:carbon monoxide (H2:CO) ratio, the reactor conditions of temperature and pressure, the nature of the catalyst and the synergy between all these factors. As shown in stoichiometric Eqns. (1) and (2), a 2:1 H2:CO ratio tends to produce water as a byproduct, whereas a 1:2 H2:CO ratio tends to produce CO2 as a byproduct. A competing reaction called the water-gas shift reaction converts carbon monoxide (CO) and water (H2O) to carbon dioxide (CO2) and hydrogen (H2); serving the triplicate function of consuming generated water vapor, increasing the H2:CO ratio and producing CO2.nCO+(2n+1)H2→CnH2n+2+nH2O  (1)2nCO+(n+1)H2→CnH2n+2+nCO2  (2)CO+H2O→CO2+H2  (3)
The Fischer-Tropsch synthesis is well-poised to serve as an alternative source of middle distillates because the reactant gas mixture utilized for the reaction, synthesis gas, is a mixture of carbon monoxide and hydrogen obtained by the conversion of carbonaceous or hydrocarbonaceous materials e.g. coal or, more commonly, natural gas; both resources being abundant compared to existing and predicted petroleum reserves. This conversion of natural gas to usable liquid fuels presents itself as an attractive synthetic route incorporating synthesis gas. Natural gas can be found as pockets of stranded gas, and on-site conversion to easily transportable and valuable liquid fuels represents a more efficient utilization of abundant natural gas. Moreover, synthesis gas can be produced from a variety of processes. Synthesis gas can be obtained from a gaseous hydrocarbon, such as methane, or from any mixtures of gaseous hydrocarbons such as in natural gas, by means of steam reforming, auto-thermal reforming, dry reforming with carbon dioxide, advanced gas heated reforming, partial oxidation, catalytic partial oxidation, or other processes known in the art. The synthesis gas so produced and the fuels eventually produced from it are substantially free of sulfurous impurities that commonly require costly removal from comparable oil and coal derived fuels.
The nature of the product mixture, e.g., distribution of molecular weights and product yield, produced from the Fischer-Tropsch process, is profoundly influenced by several reaction variables including, but not limited to, composition and morphology of the catalyst, conditions of temperature and pressure within the reactor, and the molar ratio of the gases in the reactant gas mixture.
The Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Although it is possible to directly produce middle distillate fractions containing C13–C20 hydrocarbons, an alternative is to adjust reactor conditions to favor the production of higher molecular weight products, such as C21+ hydrocarbons which tend to be waxy solids at room temperature, commonly referred to as Fischer-Tropsch wax. Subsequent processing of a Fischer-Tropsch wax may produce diesel fuel via distillation and hydrocracking processes.
Typically, in the Fischer-Tropsch synthesis, the distribution of weights that is observed can be described by likening the Fischer-Tropsch reaction to a polymerization reaction with an Anderson-Schulz-Flory chain growth probability (alpha value) that is independent of the number of carbon atoms in the lengthening molecule. The alpha value is typically interpreted as the molar ratio of Cn+1 product to Cn product.
Catalysts for the Fischer-Tropsch process typically comprise a metal selected from the group consisting of cobalt, iron, ruthenium, or other Groups 8, 9, or 10 metals from the Periodic Table of the Elements (according to the New Notation IUPAC Form as illustrated in, for example, the CRC Handbook of Chemistry and Physics, 82nd Edition, 2001–2002; said reference being the standard herein and throughout); and, optionally, at least one, and possibly more than one, promoter selected from the group consisting of the alkali metals, the alkaline earth metals, the lanthanides, copper, thorium, zirconium, rhenium, titanium, elements from Groups 13–17 of the Periodic Table; and may be supported on some carrier or unsupported.
It is well known that iron-based Fischer-Tropsch catalysts have a comparatively high water-gas shift activity, so as to ultimately produce comparatively small amount of water vapor. By contrast, reactors employing cobalt-based Fischer-Tropsch catalysts tend to produce significantly higher quantities of water vapor, owing to the relatively low water gas shift activity of the cobalt catalysts employed. Thus, reactors utilizing cobalt-based catalysts tend to produce significant amounts of gaseous water as a by-product.
Some cobalt-based Fischer-Tropsch processes place exceptionally high demands upon the mechanical, thermal and chemical properties of catalyst particles. Under reaction temperatures (200–300° C.) and pressures (20–30 bar) commonly found in cobalt-based, Fischer-Tropsch wax producing reactors, the water vapor so generated can exert a considerable water vapor partial pressure. Under these conditions, catalyst support particles, such as those comprising gamma-alumina for example, can degrade and disintegrate, providing ample opportunity for the active catalytic metal to be removed from the catalytic process via erosion and attrition processes. In addition, the alumina may react with cobalt metal to form cobalt aluminate spinels, which bind the cobalt into oxidized forms and prevent their participation in the catalytic process as zero valent metals.
Catalyst supports used in the Fischer-Tropsch synthesis are typically porous, refractory inorganic oxides. A key function of a catalyst support particle is the provision of a suitable framework by which the catalytically active metals can be deposited onto the surface of the support particle as numerous, well-dispersed clusters, thereby making the most economic use of the oftentimes costly active catalytic metal and providing for an extensive surface over which chemical reaction may occur. Thus, many techniques of catalyst synthesis strive to create support particles of high porosity and surface area upon which can be deposited the active catalytic metal. In addition to influencing the nature and concentration of active catalytic sites, properties of the catalyst support, such as surface area, pore volume, pore size and porosity, can also affect the diffusion of reactants and products to and from the active catalytic site, respectively.
The nature of the catalyst support can contribute to the effective lifetime of a catalyst, herein defined as the length of time over which the catalyst can continue to catalyze a given specified process at a practicable rate. Catalyst lifetime is governed by diverse, interrelated processes including catalyst deactivation, catalyst attrition and catalyst support degradation. Deactivation can be thought of generally as the partial or total attenuation of the ability of the catalyst to mediate the specific chemical transformation of interest and may comprise processes such as oxidation of catalytically active metal or sintering wherein distinct catalytic metal sites cluster together and begin to grow into a single crystal; and, clogging of pores in the support structure which prevents the diffusion of reactants and products to and from the active catalytic sites. Catalyst attrition can be defined as the loss of catalytic metal to the surrounding medium and eventual removal from the catalytic cycle while degradation is defined herein as a significant change in support particle morphology in response to reactor conditions, such as mechanical stress, high temperature or high water vapor partial pressure.
Gamma-alumina is a particularly important refractory, inorganic oxide of widespread technological importance in the field of catalysis, often serving as a catalyst support. Gamma-alumina is an exceptionally good choice for catalytic applications because of a defect spinel crystal lattice that imparts to it a structure that is both open and capable of high surface area. Moreover, the defect spinel structure has vacant cation sites giving the gamma-alumina some unique properties. Gamma-alumina constitutes a part of the series known as the activated, transition aluminas, so-called because it is one of a series of aluminas that can undergo transition to different polymorphs. The oxides of aluminum and the corresponding hydrates, can be classified according to the arrangement of the crystal lattice; gamma-alumina (gamma-Al2O3) being part of the gamma series by virtue of a cubic close packed (ccp) arrangement of oxygen groups. Some transitions within a series are known; for example, low-temperature dehydration of an alumina trihydrate (gibbsite, gamma-Al(OH)3) at 100° C. provides an alumina monohydrate (boehmite, gamma-AlO(OH)). Continued dehydration at temperatures below 450° C. in the gamma series leads to the transformation from boehmite to the completely dehydrated gamma-Al2O3. Further heating may result in a slow and continuous loss of surface area and a slow conversion to other polymorphs having much lower surface areas. Higher temperature treatment ultimately provides alpha-alumina (alpha-Al2O3), a denser, harder oxide of aluminum often used in abrasives and refractories; the structure of alpha-Al2O3 being less well-suited to certain catalytic applications, such as in the Fischer-Tropsch process, because of a closed crystal lattice which imparts a relatively low surface area to the catalyst particles.
Reactors that produce water vapor at high temperature and high water vapor partial pressure, such as for example, cobalt-based Fischer-Tropsch reactors producing a waxy, paraffinic hydrocarbon product, provide environments that challenge the hydrothermal and acid stability of gamma-Al2O3 supports; these supports being prone to degradation, fragmentation, phase transition or other processes that compromise the ability of the support material to adequately support catalytic metals. Thus, preparing a catalyst supported on gamma-Al2O3 of sufficient stability for use in protracted steam-producing Fischer-Tropsch reactors remains an important problem in the art.
The problem of contamination of a waxy Fischer-Tropsch product with catalyst ultra fines has been addressed in International Application WO 99/42214 wherein a catalyst support made by introducing Si, Zr, Cu, Zn, Mn, Ba, Co, Ni and/or Li as a modifying component onto and/or into an untreated catalyst support is disclosed. In particular, the modifying component is chemically bonded to the particle surfaces and/or support frameworks of the particles to suppress the solubility of the catalyst support, and prevent the factors which contribute to high catalyst attrition.
Similarly, U.S. Pat. No. 6,255,358 B1 discloses a highly stable cobalt on alumina catalyst wherein the catalyst comprises a gamma-alumina support doped with lanthanum oxide, barium oxide or a combination thereof to increase the thermal stability of the catalyst in a slurry bubble column reactor; and wherein the catalyst support employed is preferably a lanthanum or barium doped gamma-alumina support.
U.S. Pat. Nos. 5,102,851 and 5,116,879 disclose a catalyst for converting synthesis gas into a mixture of predominately paraffinic hydrocarbons wherein the catalyst includes catalytically active amounts of cobalt and a loading-insensitive second metal selected from the group consisting of platinum, iridium, rhodium and mixtures thereof composited on an alumina support wherein gamma-alumina is preferred, but a number of alumina structures, for examples, eta-alumina, xi-alumina, theta-alumina, delta-alumina, kappa-alumina, boehmite, and pseudo-boehmite can all be used as supports.
Co-pending U.S. Provisional Patent Application 60/419,021 filed on Oct. 16, 2002 and incorporated herein by reference relates to a stabilized transition alumina catalyst support of high hydrothermal stability and methods of making such a support wherein the support comprises at least one structural stabilizer selected from the group consisting of boron (B), magnesium (Mg), silicon (Si), calcium (Ca), titanium (Ti), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), strontium (Sr), zirconium (Zr), barium (Ba), selenium (Se) and the lanthanides (Lns). Catalysts made using such a support are further disclosed.
Although some in the art have attempted to solve the general problem of catalyst attrition in Fischer-Tropsch catalysts, none have offered a completely satisfactory solution to the specific problem of creating catalysts that can successfully and totally withstand the conditions within a cobalt-based Fischer-Tropsch reactor producing a hydrocarbon product comprising paraffinic wax as well as substantial quantities of high-pressure and high-temperature water vapor. In particular, alumina supports for cobalt catalysts used in such reactor, that are highly resistant to physical and chemical changes in the face of extreme acidic, hydrothermal and mechanical stresses, are still in demand within the art; the problem having not been completely solved to date. More particularly, the issues of loss of catalytic metal, collapsed particle structure, sintering of catalytically active metals including cobalt and/or platinum, and formation of cobalt-aluminate spinels in the catalysts of interest remain extant within the art.